Damage observed in the turbine section was consistent with a disruption in airflow from the compressor resulting in increased exhaust temperatures beyond the limits of the turbine components. Due to the extensive mechanical damage to the engine and compressors, the origin of the engine failure and an accurate assessment of the engine condition prior to the occurrence could not be established with certainty. The conclusions of independent analyses (by Pratt& Whitney and by the TSB Engineering Laboratory) differ regarding the origin of the damage, resulting in two scenarios that could explain the engine anomalies and subsequent engine failure.Analysis Damage observed in the turbine section was consistent with a disruption in airflow from the compressor resulting in increased exhaust temperatures beyond the limits of the turbine components. Due to the extensive mechanical damage to the engine and compressors, the origin of the engine failure and an accurate assessment of the engine condition prior to the occurrence could not be established with certainty. The conclusions of independent analyses (by Pratt& Whitney and by the TSB Engineering Laboratory) differ regarding the origin of the damage, resulting in two scenarios that could explain the engine anomalies and subsequent engine failure. Scenario One In the first scenario, as indicated by TSB Engineering Laboratory report LP114/04, it is likely that the engine failure was initiated when an 8th stage rotor airfoil dislodged from its respective disk slot and contacted an adjacent stator. Supporting this scenario is physical evidence suggesting that the discoloured airfoil from the 8thstage rotor had dislodged prior to the engine failure. As the exact amount of time that the airfoil had been dislodged before the engine failure could not be determined, no categorical conclusion could be drawn. However, it is unlikely that the airfoil was dislodged for the entire 3.5hours of engine operation following the original compressor surging. Also, the reason for the airfoil liberation from the disk could not be determined. In this scenario, the most likely reason is that the airfoil retaining pin failed. A search of the loose debris submitted with the high-pressure compressor module did not locate any of the retainer. The retainer, because of its small size, could follow the gas stream and exit the engine. Material the size of the retainer could damage airfoils in the downstream path and cause compressor surging. This surging could result in cyclic loading of the 8thand 9thstage rotor airfoils, and failure of the aged PRBCvalve. The general damage pattern to the LPCcompressor stages, including the 3rdstage, indicates that some of the LPCstages also experienced conditions leading to a widespread fatigue cracking of the airfoils. In this scenario, this fatigue cracking would be secondary to, and a result of, the failure of the 8th stage rotor. Scenario Two The second scenario focuses on the 3rdstage rotor. The number of 3rdstage rotor airfoils that exhibited fatigue is consistent with significant vibratory excitement of that stage. Prior to the engine failure, a liberated piece of material, either just preceding or just following the 3rdstage, could travel aft, inducing damage to subsequent compressor stages and resulting in reduced compressor stability. The vibratory excitement caused by the material transiting the compressor sections could result in the failure of a 3rd stage rotor airfoil and the subsequent engine failure. Summary The ECM data recorded on the day of the occurrence indicated that there was an inefficiency within the compressor. However, it is unlikely that the impending failure could have been predicted, given the intended purpose of the ECMprogram. Furthermore, no discernable changes in previous trended parameter levels suggested an emerging problem. One or two more days of ECMdata would have had to be entered before it would have been possible to know if the outliers were the beginning of some parameter shift or simply normal data variation. In either scenario, damage from loose compressor hardware resulted in a marginally compromised compressor. The physical evidence in both the 3rdand 8thstage rotors indicates that compressor surging and vibratory excitement may have contributed to the ultimate failure of the engine. At a minimum, the compressor stalls were symptoms of an existing gas path inefficiency. The replacement PRBCvalve, due to its slightly different operating characteristics and the inaccuracies in the JT8Dtroubleshooting flowchart, masked the compromised compressor. During troubleshooting to determine the cause of the compressor surges, a boroscopic inspection was not performed, nor was one required. Had a boroscopic inspection been performed, it is possible that missing or damaged compressor hardware would have been detected. The following TSB Engineering Laboratory reports were completed: LP 45/03 - LP Compressor Stage Failure, and LP 114/04 - HP Compressor Stage Failure. These reports are available from the Transportation Safety Board of Canada upon request. The compressor gas path was compromised either by the failure of an 8thstage airfoil retaining pin or by a liberated stator in close proximity to the 3rd stage rotor. Airflow instabilities resulted in vibratory excitement of the 3rdand 8thstage rotors leading to widespread fatigue cracking in the rotor airfoils. As a result of the compressor airfoil fatigue cracking, either a 3rdor 8thstage rotor airfoil was liberated during the initial climb, resulting in severe engine damage and loss of engine power.Findings as to Causes and Contributing Factors The compressor gas path was compromised either by the failure of an 8thstage airfoil retaining pin or by a liberated stator in close proximity to the 3rd stage rotor. Airflow instabilities resulted in vibratory excitement of the 3rdand 8thstage rotors leading to widespread fatigue cracking in the rotor airfoils. As a result of the compressor airfoil fatigue cracking, either a 3rdor 8thstage rotor airfoil was liberated during the initial climb, resulting in severe engine damage and loss of engine power. The Pratt & Whitney JT8D Maintenance Manual does not have a warning about potential compressor airfoil fatigue cracking that may develop as a result of compressor stall or surge. Consequently, maintenance personnel may be unaware that compressor airfoil fatigue cracking can occur as a result of a compressor surge. The troubleshooting flowchart in the Pratt & Whitney JT8D Maintenance Manual omits any reference to a test procedure that should be performed after replacing a pressure ratio bleed control (PRBC) valve. This omission, combined with a PRBCvalve with slightly different operating characteristics, may result in the compressor appearing to be stable when it is not. WestJet maintenance personnel replaced the PRBC valve and the start bleed control valve in accordance with the Pratt& Whitney JT8D Maintenance Manual. A boroscopic inspection was not carried out nor was it required by regulation or procedure and, as a result, compressor gas path anomalies that might have been detected were not.Findings as to Risk The Pratt & Whitney JT8D Maintenance Manual does not have a warning about potential compressor airfoil fatigue cracking that may develop as a result of compressor stall or surge. Consequently, maintenance personnel may be unaware that compressor airfoil fatigue cracking can occur as a result of a compressor surge. The troubleshooting flowchart in the Pratt & Whitney JT8D Maintenance Manual omits any reference to a test procedure that should be performed after replacing a pressure ratio bleed control (PRBC) valve. This omission, combined with a PRBCvalve with slightly different operating characteristics, may result in the compressor appearing to be stable when it is not. WestJet maintenance personnel replaced the PRBC valve and the start bleed control valve in accordance with the Pratt& Whitney JT8D Maintenance Manual. A boroscopic inspection was not carried out nor was it required by regulation or procedure and, as a result, compressor gas path anomalies that might have been detected were not. The damage noted to the PRBC components is typical of a valve that has been in service for some time and has been subject to an unusual and rapid change in air pressures, as seen during a compressor surge or stall.Other Finding The damage noted to the PRBC components is typical of a valve that has been in service for some time and has been subject to an unusual and rapid change in air pressures, as seen during a compressor surge or stall.